He and his wife Justyna, then had to wade waist-deep through ice-cold water to gain access to the mesmerizing cave.
But after pitching up a tent and spending the night in the cave, Lars managed to capture these stunning images before exploring the depths in a 1km walk through the pitch-black.
Lars said: "My general goal is to explore the planet as much as possible and I'd never been to Vietnam before. "It is hard work to access the cave, because there is no road nearby. The only way to reach the cave is to walk 10 kilometers through the jungle, uphill and downhill.
"You need to cross several rivers with a water level up to 1.5 meters and you'll be attacked by dozens of leeches who are attaching themselves to your skin. This is certainly not to everybody's taste.